Several anacardic acids and their analogues were synthesised and their plactamase inhibitory activities were determined. The structure-activity relationships suggested by these data are discussed.The most important mechanism by which bacteria become resistant to Qlactam antibiotics (for example penicillins, cephalosporins and carbapenems) is the production of ~lactamases which catalyse the hydrolysis of the plactam bond thereby inactivating the antibiotic. In recent year8 there has been a growing number of B_lactamase-producing bacterial clinical isolates which are resistant to many of the available ~lactam antibioticsl. There are two approaches to overcoming Qlactamase mediated resistance: i) development of ~lactam antibiotics which are not hydrolysed by these enzymes and ii) co-administering the antibiotic with a ~lactamase inhibitor to prevent antibiotic hydrolysis. The second approach has resulted in the development of amoxycillin and clavulanic acid combination as a successful clinical agent2* As part of our continuing research effort to find new P_lactamase inhibitors, we have recently reported3 that 6-(heptadecatrien-8',1 l',lC-yl)-Zhydroxy benzoic acid (SB-202742, l), a 17:3 anacardic acid4 displayed significant B_lactamase inhibitory activity. We now report on the chemical synthesis and biological properties of a number of derivatives of 1 and discuss the structureactivity relationships that are suggested by these data.